What Is a Completion Handler in Swift Programming Language?

A completion handler in Swift is a function that calls back when a task completes. This is why it is also called a callback function.

A callback function is passed as an argument into another function. When this function completes running a task, it executes the callback function.

Example :

Here is a demonstrative (but not useful) example of using a completion handler in Swift:

func greet(_ completion: () -> ()) {
    print("Hello world!")
    completion()
}
greet({
  print("Task finished.")
})

Output :

Hello world!
Task finished.

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